1. Field of the Invention
This invention relates in general to well drilling and, in particular, to systems, program products, and methods associated with controlling drilling-fluid parameters in an oil or gas well.
2. Description of the Related Art
More and more oil exploration is moving toward ever challenging environments, which present increasing environmental and technical risks. Such environments are resulting in narrow or margins between the pressure of fluids inside the pores of rock at the bottom of a well hole, known as pore pressure, and the pressure which causes a rock formation containing or adjacent a formation containing desired hydrocarbons to fracture, known as the fracture or leak off pressure of the formation. Well drilling systems can include a drilling rig located substantially at the surface. A drill string positioned within the casings extends through the casings to the formation containing hydrocarbons. The drilling string and annular area between the drilling string and between the wellbore and inner-most casing, referred to as the annulus, form a drilling circulation system.
Primary and intermediate casings (strings) are cemented inside a drilling hole to prevent direct transmission of fluid pressure to intermediate formations. The casing strings are designed for operationally limiting gradients, on the high side for overburden, fracture, borehole stability, etc. and on the low side for pore pressure control and/or wellbore integrity, etc. The overburden gradient is initially quite low and increases in a highly non-linear fashion with depth. Fracture gradient follows a similar trend, with separation from the overburden gradient diminishing with depth. Pore pressure increases with depth, details of which depend upon conditions in each formation penetrated. Separation of the upper limit (overburden or fracture) and lower limiting pore pressure is used to determine the number and depth of casing strings to be run.
As well drilling operations reach into deeper and deeper depths, proper well control becomes ever more challenging and yet more critical. Variations in the density of the drilling fluid resulting in more pronounced changes in hydrostatic pressure at the bottom of the well bore. Further, and deeper depths, some formations may not tolerate significant variations in hydrostatic pressure. Such variations in hydrostatic pressure can result in either a formation fluid influx into the wellbore, known as a “kick,” or a loss of drilling fluid to the formation, known as “lost circulation.”
Drilling a well bore generally requires circulating a drilling fluid through the drilling fluid circulation system. At the surface, the drilling fluid is pumped through a flowmeter and down the drilling string to the bottom hole of the well and is returned via the annulus. The fluid exits the annulus through a return line, outlet flowmeter or flowmeters, degasser, shale shaker to remove drilling clippings, and into a fluid storage tank to again be pumped down the drilling string. A choke in the return line can be used to control pressure within the annulus.
As the drilling fluid is circulated through the circulation system under a positive pressure from a surface “mud” pump (or bottom hole pump), the drilling fluid encounters a loss in pressure due to friction, known as “circulating friction.” The circulating friction is generally the result of an interaction between the drilling fluid and the inner surface of the drilling fluid conductors through which the drilling fluid is circulating. The mud pump and bottom hole circulating pressure is generally kept substantially constant for a particular set of operating parameters. When the drilling fluid is not being circulated, the bottom hole pressure exerted on the formation is a non-circulating or “static” hydrostatic pressure equal to the hydrostatic weight of the drilling fluid column. When drilling and under steady-state conditions, the drilling fluid is circulated and the bottom hole hydrostatic pressure exerted on the formation is increased above the non-circulating or “static” hydrostatic pressure by the amount of friction pressure in the well bore annulus. The resulting bottom hole pressure applied to the formation when circulating drilling fluid is known as the equivalent circulating density or “ECD.”
The drilling fluid is utilized to provide hydrostatic well control. In overbalanced drilling the weight of the drilling fluid and the setting of the choke is selected so that the dynamic pressure at the lower ends of the drilling and casing strings are greater than the pore pressure, but less than the fracture or leak off pressure. In near balanced drilling the dynamic pressure is maintained approximately the same as the pore pressure. In under balanced drilling, the dynamic pressure is maintained less than the pore pressure. In each type of drilling, the dynamic pressure is maintained by a combination of the drilling fluid weight (density) and control of the choke via surface well control equipment.
In order to determine if a “kick” is being encountered or if there is lost circulation, mass flow and/or volume flow can be monitored both in and out of the system to detect an influx or loss of mass or volume of the drilling fluid or by means of downhole temperature sensors, downhole hydrocarbon sensors, pressure chain sensors, or pressure pulse sensors. A discrepancy between predicted and monitored flow out can be indicative of an influx into or loss of the drilling fluid. The difference in mass being supplied to the drilling string and returned from the well annulus provides an indication of whether or not fluid is entering or exiting downhole. If a discrepancy is detected, the bottom hole pressure is controlled by a process known as managed pressure drilling.
Most recent developments in drilling systems include those described in U.S. Pat. No. 6,352,129 by Best titled “Drilling System,” U.S. Pat. No. 6,374,925 by Elkins et al. titled “Well Drilling Method and System,” U.S. Pat. No. 6,484,816 by Koederitz titled “Method and System for Controlling Well Bore Pressure,” and WIPO Patent Document No. WO 02/50398 A1 by Leuchtenberg titled “Closed-Loop Fluid Handling System for Well Drilling.”
According to one methodology, weighing agents, e.g., barite, are added to the drilling fluid to increase the “weight” in response to influx or oil or other low density material is added to the drilling fluid in response to fluid loss to set a desired drilling fluid density to change the equivalent circulating density and bottom hole pressure. This methodology is extremely inefficient as hours may pass as the weighing agent is being added to the drilling fluid and circulated through the circulation system. Another methodology of adjusting bottom hole pressure in response to an influx or drilling fluid loss includes adjusting the fluid choke in the fluid output conductor when circulating the drilling fluid and/or when drilling to apply sufficient back pressure. Another methodology of adjusting bottom hole pressure includes injecting fluid into the annulus when not performing drilling.
In order to function, each methodology incorporates assumptions used in monitoring pressure, volume, and density entering and exiting the circulation system and in determining desired drilling fluid density adjustment parameters or choke configuration parameters. These assumptions include the drilling fluid being a single-phase liquid that is incompressible. The assumptions also include the mud pump pressure being substantially constant. The assumptions further include that the flowrate of the drilling fluid entering the drilling string from the surface, although adjustable, is substantially constant. In the latter two methodologies, these assumptions also include that the density, although adjustable, is substantially constant.
Methodologies employed in the state-of-the-art for managing bottom hole pressure, general known as managed pressure drilling, do not account for, i.e., ignore, the pressure changes inside the drilling string along with other significant factors in the whole system that contribute in substantial ways to operational effects in the annulus, at the choke, at the bottom of the hole. Previously employed methodologies do not account for the compressibility of associated rocks, fluid in the rocks, cement in the hole, the casing strings cemented in the hole, the drilling fluid, the drilling string assembly when drilling, which is an enormous volume of material. The volume to pressurize the circulation system is small but it is not zero. Additionally, recognized by the Applicant is that adjusting the choke in the output line adjusts annulus pressure, but not necessarily pressure within the drilling string.
Therefore, there is still a need for a system, program product, and methods for enhanced dynamic control of drilling fluid pressures and parameters. Particularly, recognized by the Applicant is the need for a system that can monitor and control pressure, volume, density, temperature, fluid composition, molecular concentration of both single phase and multiphase drilling fluid both when entering and when exiting the drilling circulation system and at any location from the surface and along the length inside the drilling string and in the annulus, i.e., either side of the U-tube, at any time or operational drilling phase. Recognized also is the need for a system that can account for the pressure changes and other factors inside the drilling string, in the annulus, at the choke, at the bottom of the hole, and that can account for the volume of drilling fluid required to pressurize the circulation system. Recognized also is the need for a system that can measure compressibility of associated rocks, fluid in the rocks, cement in the hole, the casing strings cemented in the hole, the drilling fluid, the drilling string assembly to formulate a running description of the physical behavior of the drilling system and all components, and that can account for such compressibility to thereby enhance dynamic density control throughout the system. Recognized also is the need for a system that can account for friction losses for any location for any rheology and physical dimensions of the circulation system and that can determine and compensate for the existence of mud channels in the drilling string cement. Recognized further is the need for a system that can dynamically manipulate the mud weight window, and that can predict maximum dynamic bottom hole pressure at future depths to be drilled to thereby anticipate future drilling requirements to drill at the future depth including a requirement to order supplies, people, third party services, etc. Recognized further, also, is the need for a system that can add gas or other fluids to drilling fluid and account for gas or other fluids added in the drilling fluid.